The Study On Alkali-Catalyzed Gasification Of Coal Char:Behaviors Of Hydrogen Production And Performances Of The Catalysts

It is momentous to investigate the gasification of coal at low temperature for it enjoys numerous advantages, such as lowering the gasification temperature, promoting the efficiency of gasification, producing the gases rich in hydrogen, decreasing the subsequent shift processes and diminishing the cost. The alkali metal was adopted in this research focusing on the behaviors of hydrogen production and the performances of the catalysts to investigate the effect on the selectivity of gases production in order to enhance the hydrogen production and minimize the methane formation. Then we proposed that the alkali metal deactivation in the steam gasification could be retrained by adding the calcium additives during the charring stage. Different kinds of calcium additives and catalysts were selected to further study the deactivation inhibited by the additives and the interaction among the additives, the catalysts and the coal char. The effects of the way of adding the additives were also investigated. The kinetic models were compared about the fitness between the gasification with or without catalysts.(1)Steam gasification of coal char catalyzed by potassium carbonate was investigated on a laboratory fixed bed reactor to examine the catalytic effects not only on the reaction rate but also on the reaction selectivity. It was observed that the catalytic gasification of coal char with steam occurred significantly in a temperature range of700-750℃, producing a hydrogen-rich gas with slight formation of carbon monoxide and virtually no formation of methane. An oxygen transfer and intermediate hybrid mechanism of the catalytic char gasification with steam is proposed for understanding of the experimental data regarding both the kinetic behaviors and reaction selectivity. The study has highlighted the advantages of the catalytic gasification of coal char over the conventional coal gasification with respect to the reaction selectivity. The catalytic steam gasification of coal char makes it possible to eliminate or simplify the methane reforming and water-gas shift processes in the traditional gas-to-hydrogen purification system.(2)A novel approach has been proposed for mitigating the potassium deactivation in the K2CO3-catalyzed steam gasification of coal char by addition of Ca(OH)2in the char preparation. It was experimentally found that the Ca(OH)2-added char had higher reactivity for the catalytic gasification than the raw char. Ca(OH)2played a role in suppressing the interactions of K2CO3with acidic minerals in coal during the gasification and also probably in forming more active oxygenated intermediate on the char surface. An oxygen transfer and intermediate hybrid mechanism is applied for understanding of the rate and selectivity of the catalytic gasification.(3)Experiments were carried out to investigate the K2CO3-catalyzed gasification performances for varying chars prepared from four coals with prior addition of calcium additive and without calcium additive. Three calcium species, Ca(OH)2, Ca(Ac)2or Ca(CH3COO)2, and CaCO3, were used in the study. Experiments were also conducted in an attempt to achieve the understanding of the effect of calcium additive on the potassium deactivation, as well as the interactive effect between potassium, calcium and char. It was found that each calcium additive acted as a deterrent to the potassium deactivation and thus promoted the catalytic gasification, and the magnitudes of promotion varied depending on coal and calcium species. Ca(OH)2and Ca(Ac)2were more effective to inhibit the potassium deactivation than CaCO3. Furthermore, potassium together with calcium showed a mineral-unrelated synergy towards the char gasification. A bimetallic carbonate, K2Ca(CO3)2, which formed even when the chars prepared from coal with either Ca(OH)2or Ca(Ac)2was subsequently mixed with K2CO3, was likely to contribute to the synergistic effect.(4)This part investigated the effects of the different loadings of Ca(OH)2during the pyrolysis process on the catalytic gasification of different coal samples. For the coals rich in minerals, the increasing loading could obviously protect the catalyst. But for the coals poor in minerals, the effect of protection was not so sharp. Then the effects of different catalysts, including Li2CO3and K2CO3, on the gasification of different coal samples were studied with different loadings of Ca(OH)2. For the coals rich in minerals, Li2CO3favored more interaction with the minerals and resulted the rate of gasification was relatively slow. From the X-ray diffraction results of the ash, the peak of LiAlSiO4, which was formed of the interaction between lithium and the acidic minerals in Jinyou coal, was found notable. In contrast, it was not so obvious when the gasification was catalyzed by K2CO3. After Ca(OH)2was added, the lithium was existed not only in LiAlSiO4but also in Li2CaSiO4. For the coals rich in minerals, the catalytic effect of Li2CO3was less than K2CO3no matter the additive was added or not. For the coals poor in minerals, the rate of gasification catalyzed by Li2CO3was faster than that catalyzed by K2CO3.(5)Hydrothermal pretreatment of coal with the addition of Ca(OH)2was found more effective for promoting the K2CO3-catalyzed char gasification than the physical addition way for both JY anthracite coal and HB bituminous coal used. The effect of hydrothermal pretreatment was more remarkable for JY coal which suffers significant catalyst deactivation due to its high mineral content. For this type of coal, employing more severe hydrothermal pretreatment conditions was proven to significantly enhance the gasification rate. It was observed that kaolinite and quartz in coal hydrothermally reacted with calcium forming hydrated calcium aluminosilicates, which, unlike the original minerals, was inactive for the deactivation reactions of potassium. Consequently, the hydrothermal pretreatment allowed more potassium to persist as a water-soluble entity during the gasification. This was a main mechanism underlying the promoted catalytic gasification by the hydrothermal treatment.(6)The adopted kinetic models included harmonious model (HM), shrink core model (SCM), blending model (BM), random pore model (RPM), modified volume model (MVM) and extended modified volume model (EMVM). The typical gasification models with or without the catalysts were fitted by the aforesaid models. The gasification without catalysts was better fitted by HM, SCM and BM than the gasification with catalysts. Both of the typical gasification models were well fitted by SCM, MVM and EMVM. Although the square value of correlation indexes of HM and SCM were larger than that of RPM, the activation energy calculated by HM and SCM were close to that calculated by the RPM. The square value of correlation index of BM were less than that of RPM, however the activation energy calculated by BM were far away from that calculated by RPM.